Servo self-write disk drive with dual-stage actuator

ABSTRACT

A technique for setting the initial servo track pitch for a servo system of a hard disk drive (HDD) is based on using a secondary actuator, such as a microactuator or a milliactuator. The actuator of the HDD is positioned against a crash stop and a burst pattern is written on a hard disk while the read/write head is in a first position. A bias voltage of the secondary actuator is incrementally changed to change the position of the read/write head and a burst pattern is written for each change. The overlap is determined as a sum of the averaged amplitudes of the burst patterns that are adjacent to a selected burst pattern divided by the averaged amplitude of the selected burst pattern. The process is terminated when the determined overlap for each selected burst pattern is within a selected criterion of a target overlap value.

RELATED APPLICATION

This application is related to concurrently filed co-pending applicationtitled: “SERVO SELF-WRITE DISK DRIVE WITH DUAL-STAGE ACUTATOR” Ser. No.______ (Applicants' Docket HSJ920030123US1)

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to hard disk drives (HDDs). Moreparticularly, the present invention relates to a technique for settingthe initial servo track pitch for a Self-Servo Writing (SSW) process.

2. Description of the Related Art

FIG. 1 shows an exemplary hard disk drive (HDD) 100 having a magneticread/write head (or a recording slider) 101 that includes, for example,an offset head, that is positioned over a selected track on a magneticdisk 102 using a dual-stage servo system for writing data to and/orreading data from disk 102. Data is recorded in arrays of concentricdata information tracks on the surface of disk 102. While HDD 100 showsonly a single magnetic disk 102, HDDs typically have a plurality ofstacked, commonly rotated rigid magnetic disks.

The dual-stage servo system of HDD 100 includes an actuator 105, avoice-coil motor (VCM)104, for coarse positioning a read/write headsuspension 106, and a secondary actuator, such as a microactuator ormicropositioner, for fine positioning read/write head 101 over aselected track. As used herein, a microactuator (or a micropositioner)is a small actuator that is placed between a suspension and a slider andmoves the slider relative to the suspension.

FIG. 2 depicts a cross-section of an exemplary suspension andmicroactuator arrangement 200 that includes a suspension 201, amicroactuator 205 and a slider 209. Suspension 201 includes a load beam202, a dimple 203 and a flexure 204. Microactuator 205 includes asubstrate 206, a microactuator structure 207 and at least one flexureelement 208. Substrate 206 is the stationary structure of microactuator205. Microactuator structure 207 is the movable structure ofmicroactuator 205. Slider 209 includes a read element 210 and a writeelement 211 that is offset from read element 201.

Track accessing and following is typically provided by a servo system ofan HDD by using magnetically written patterns, referred to as servopatterns, that are stored on at least one magnetic disk of an HDD. Onecommon type of servo pattern arrangement that is used is referred to asa dedicated servo system in which one surface of one of the hard disksis used for storing all the servo patterns. Another common type of servopattern arrangement that is used is referred to as a sector servo systemin which a small portion of a track between each sector or betweenseveral sectors on each track on each data surface is used for storingthe servo patterns. Yet another common type of servo pattern arrangementthat is used is referred to as a hybrid servo system in which bothdedicated and servo-sector type servo system techniques are used,thereby obtaining advantages of each respective type of servo system.

One technique that is used for writing servo patterns on the disk ordisks of an HDD uses special equipment referred to as a servo writersystem. A servo writer system includes, for example, a laser-measuredaccess system for accurately positioning the heads of the servo writersystem over the disks of the HDD for accurately writing the servopatterns. The HDD is clamped to a servo writer in order to maintainaccurate positioning between the HDD and the servo writer. U.S. Pat. No.6,519,107 B1 to Ehrlich et al. discloses an exemplary a technique forwriting servo patterns onto a magnetic hard disk drive.

One drawback of using a servo writer system is that it must be used in aclean environment in order to reduce the probability of contamination ofthe HDD because the HDD is open during the servo pattern writingprocess. Additionally, the resonances of the HDD change when the HDD isunclamped from the servo writer. Consequently, the servo system of theHDD does not perfectly follow the servo patterns, resulting inrepeatable runout that makes determination of being on-track difficultby the servo system.

Self-servo writing (SSW) techniques have been developed for reducing thedrawbacks associated writing servo patterns using a servo writer. Forexample, U.S. Pat. No. 6,040,955 to Brown et al. relates to a self-servowriting (SSW) technique in which servo information is written on amagnetic disk starting at a first crash stop of an HDD. The head writingthe servo information is moved toward the other crash stop until thedetected amplitude of the just-written servo information equals apredetermined amount, at which point more servo information is written.Movement of the head and writing of the next servo pattern continuesacross the disk until the second crash stop is encountered. U.S. Pat.No. 6,429,989 B1 to Shultz et al. relates to an SSW technique thatwrites timing marks across the surface of a magnetic disk based ondetecting both the passage of the timing marks and writing radialextensions to timing marks at substantially the same circumferentialpositions.

One aspect of an SSW process is that the initial servo track pitch isset at the beginning of the SSW process. The compliance of the InnerDiameter (ID) crash stop and a predetermined amount of VCM current areused for producing a set of equally spaced tracks in a radial directionthat are used as the basis for the radial propagation across the entiresurface of the disk during the next phase of the SSW process. FIG. 3shows a flowchart 300 for an exemplary conventional initial servotrack-pitch-setting technique that is performed at the beginning of aconventional SSW process. At step 301, the motor driving the magneticdisks of the HDD is driven at the desired servowriter speed. At step302, the actuator is unlatched from the ramp and the read/write headsare loaded onto the disk surface at a controlled speed. At step 303, theactuator is biased so that the read/write head is against the ID crashstop and the actuator is made ready for the SSW process. At step 304,burst patterns are written using a predetermined VCM current for apredetermined number of tracks, such as 16 tracks. Usually, 100-200bursts are written per one disk revolution. For example, if the trackhas 200 sectors (sectors 0-199), a burst in written in each of sectors0-199. The burst write timing and the VCM current are changed for eachservo track.

FIG. 4 depicts the result of the exemplary servo initialtrack-pitch-setting technique shown in FIG. 3 after bursts are writtenfor 16 tracks. FIG. 4 shows 16 servo tracks of two sectors, sectors 0and 1. The lower portion of FIG. 4 is at the ID of the disk and theupper portion is toward the OD of the disk. Bursts b0-b15 have beenwritten in each sector 0 and 1. Disk rotation is from right to left.

After the burst pattern has been written, the head is moved toward theinnermost portion of the disk at step 305 and burst b0, i.e., the burstpattern located closest to the ID of the disk, is located using the readsensor of the read/write head. At step 306, the read/write head is movedtoward outer diameter using very small steps of VCM current and burstsb1 and b2 are located. At step 307, the read sensor portion of the headis positioned over the center of burst b1 so that the amplitude of burstb0 equals the amplitude of burst b2 and the amplitude of burst b1 is amaximum. At step 308, the respective amplitudes of bursts b0, b1 and b2are measured during several disk revolutions and averaged.

At step 309, the overlap is calculated, defined as Overlap=(b0+b2)/b1,in which b0, b1 and b2 are the respective averaged amplitudes of burstsb0, b1 and b2. At step 310, the head is moved toward the OD of the diskmeasuring and averaging the amplitudes of each burst b2-b14 and theirrespectively adjacent bursts, and calculating the overlap similar to theoverlap calculation defined in step 309. For each measurement in step310, the read sensor portion of the head is positioned over the centerof the burst for which the overlap measurement is being made (i.e., thecenter of each burst b2-b14), so that the amplitudes of the bursts thatare adjacent to the burst being measures are equal and the amplitude ofthe burst being measured is a maximum. At step 311, the calculatedoverlaps are compared to a target overlap value, such as 0.9. If, atstep 311, the difference between the calculated overlaps and the targetoverlap value is within a selected criterion, such as 2%, the flowcontinues to step 312 and the initial track-pitch-setting technique isterminated.

If, at step 312, the calculated overlap is not within the selectedcriterion of the target overlap value, then flow continues to step 313where it is determined whether the calculated overlap is greater thanthe target overlap value. If, at step 313, the calculated overlap isgreater than the target overlap value, flow continues to step 314 wherethe predetermined VCM current interval is increased an increment. Flowcontinues to step 315. If, at step 313, the calculated overlap is lessthan the target overlap value, flow continues to step 316 where thepredetermined VCM current overlap is decreased an increment. Flowcontinues to step 315 where all previously written bursts are erased.Flow continues to step 303 with the new predetermined VCM current andthe process is repeated until the calculated overlap is within theselected criterion of the target overlap value.

At the end of initial servo track setting process of FIG. 3, a set ofequally spaced tracks in the radial direction (i.e., 16 tracks having afew hundreds of burst patterns) have been created. The patterns arelocated at inner diameter portion of the disk. A conventional SSW usesthe initial track-pitch-setting technique for compensating for a headhaving a large read/write offset. That is, the edge of the actuatortouches the ID crash stop so no servo control is necessary for placingthe head at the center of each burst. The number of written tracks mustbe greater than 2+ Read/Write offset of the head in tracks because theread/write offset of the head is much greater than the servo trackpitch, as depicted in FIG. 5. In FIG. 5, a head 501 includes readelement 502 and write element 503, which are separated by read/writeoffset 504. Bursts 505 are shown written on servo tracks 506.

The conventional initial servo-track-pitch-setting technique relies onID crash stop compliance for providing controllable, open-loop movementof the head. That is, when the actuator is pushed against the ID crashstop, i.e., step 303 in FIG. 3, the ID crash stop is compressed and theposition of the head different than if the ID crash stop were notcompressed. The position of the head when the ID crash stop iscompressed is related to the amount of VCM current that is used forcompressing the ID crash stop. FIG. 6 is a graph depicting therelationship between crash stop force (pushing force) in terms of VCMcurrent as a function of actuator position. The VCM current values shownin FIG. 6 are representative and can change depending on the materialused for the ID crash stop, the VCM torque constant, the geometry of theID crash stop and the actuator, and other external forces. As FIG. 6shows, ID crash stop compression, i.e., the position of the actuator,and the applied force are not linearly related. Typically, thecompression range indicated by 601 is used during an initial servo tracksetting process. The curve of FIG. 6 is not exactly repeatable so headposition at the ID crash stop is not exactly repeatable for the same VCMcurrent. As a result, when a conventional initial servo track settingprocess is repeated, the process takes a lot of time. Further,additional time is required for the position of the head to settle asthe head is moved across the disk based on VCM current. Moreover, theremainder of the SSW process is performed using the final predeterminedfixed VCM current so the overall processing time and positional accuracyof the servo information can be adversely affected. Accordingly,expensive materials must be used for the ID crash stop because the IDcrash stop compliance characteristics are critical for the initialtrack-pitch-setting technique.

Consequently, what is needed is a technique for setting the initialservo track pitch that does not rely on ID crash stop compliancecharacteristics as a basis for setting servo track pitch.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a technique for setting the initial servotrack pitch that does not rely on ID crash stop compliancecharacteristics as a basis for setting servo track pitch. Further, thetechnique of the present invention improves the accuracy of the overallSSW process.

The advantages of the present invention are provided by a method forsetting an initial servo track pitch for a servo system of a hard diskdrive having at least one hard disk, an actuator, a secondary actuator,such as a microactuator or a milliactuator, a read/write headcorresponding each hard disk, and at least one crash stop for theactuator. The actuator is positioned against the crash stop, either aninside diameter crash stop or an outside diameter crash stop, and aburst pattern is written on at least one hard disk while the actuator ispositioned against the crash stop and while the read/write head is in afirst position. The bias voltage that is applied to the secondaryactuator is changed by a predetermined bias voltage increment for apredetermined number of times, such as sixteen times, to change theposition of the read/write head a corresponding number of times and aburst pattern is written on at least one hard disk at each respectivechanged position of the read/write head. An amount of overlap isdetermined for at least one selected burst pattern having two burstpatterns that are adjacent to the burst pattern. When sixteen burstpatterns are written, fourteen burst patterns are typically selected fordetermining the overlap. The amount of overlap is determined bymeasuring and averaging the amplitude of a plurality of selected burstpatterns, such that each selected burst pattern has two adjacent burstpatterns, and by measuring and averaging the amplitude of each burstpattern that is adjacent to each selected burst pattern. The overlap isthen determined for each selected burst pattern as a sum of the averagedamplitudes of the burst patterns that are adjacent to the selected burstpattern divided by the averaged amplitude of the selected burst pattern.The method for setting the initial servo track pitch is terminated whenthe amount of overlap determined for each selected burst pattern iswithin a selected criterion of a predetermined target overlap value.Otherwise, the predetermined bias voltage increment is increased whenthe amount of overlap for each selected burst pattern is greater thanthe predetermined target overlap value, or decreased when the amount ofoverlap for each selected burst pattern is less than the predeterminedtarget overlap value. The process is repeated with the new predeterminedbias voltage increment.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not bylimitation in the accompanying figures in which like reference numeralsindicate similar elements and in which:

FIG. 1 shows an exemplary hard disk drive (HDD) having a dual-stageservo system;

FIG. 2 depicts a cross-section of an exemplary suspension andmicroactuator arrangement;

FIG. 3 shows a flowchart for an exemplary initial servotrack-pitch-setting technique that is performed at the beginning of aconventional SSW process;

FIG. 4 depicts the result of the exemplary servo initialtrack-pitch-setting technique shown in FIG. 3 after bursts are writtenfor 16 tracks;

FIG. 5 depicts the read/write offset of an exemplary head;

FIG. 6 is a graph depicting the relationship between crash stop force(pushing force) in terms of VCM current as a function of actuatorposition; and

FIG. 7 shows a flowchart for an exemplary initial track-pitch-settingtechnique according to the present that is performed at the beginning ofan SSW process.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a technique for setting the initial servotrack pitch for a dual servo system HDD that does not rely on ID crashstop compliance characteristics as a basis for setting servo trackpitch. The present invention uses a secondary actuator for setting theinitial servo track pitch setting. While any type of secondary actuator,such as a microactuator or a milliactuator, can be used with the initialservo track pitch setting technique of the present invention, thefollowing description of the present invention is based on use of amicroactuator.

The present invention utilizes the stroke of a microactuator for settingthe initial servo track pitch. A single predetermined fixed VCM currentis used for pushing the actuator against ID crash stop, and the VCMcurrent remains the same during the SSW process. Several differentmicroactuator bias voltages, for example, 16, are used for positioningthe head for the write/read process of the initial servo track pitchsetting operation, and an erase operation, if necessary.

Currently available microactuators have a stroke of approximately ±1 μmfor a ±30 V bias voltage. The relationship between the stroke and theinput bias voltageis linear and has excellent repeatability. Currentlyavailable HDDs have approximately 100K TPI with a track pitch for a datatrack that is equal to 0.25 μm. Thus, the stroke of a currentlyavailable microactuator covers eight data tracks, i.e., 2 μm/0.25 μm=8).In a conventional SSW process, the servo track pitch is a half of thedata track pitch. Accordingly, a microactuator stroke covers 16 servotracks. Further, the typical read/write offset of a head for a currentlyavailable HDD is between 3 to 8 servo tracks. Thus, currently availablemicroactuators can be used for an SSW process according to the presentinvention for currently available HDDs. Moreover, the stroke of amicroactuator can be expanded beyond 16 servo tracks by increasing therange of the bias voltage that is applied to the microactuator. Thus, asthe TPI capability of HDDs increases and the track pitch becomeaccordingly narrower, microactuators will have sufficient performance toprovide an initial servo track setting operation for an SSW processaccording to the present invention.

The linear relationship between a microactuator stroke and bias voltageutilized by the present invention makes it easy to calculate thepredetermined fixed bias voltage for the next step. Additionally, thetime required for head settling is very short so overall SSW processingtime is reduced accordingly. Further, the stoke of a microactuator hasexcellent repeatability as a function of bias voltage, so processingbecomes more accurate and can be done in an overall shorter period oftime. Further still, ID crash stop compliance characteristics are not asimportant for the present invention so cheaper materials can be selectedfor the ID crash stop.

FIG. 7 shows a flowchart 700 for an exemplary initialtrack-pitch-setting technique according to the present that is performedat the beginning of an SSW process. At step 701, the motor driving themagnetic disks of the HDD is driven at the desired servowriter speed. Atstep 702, the actuator is unlatched from the ramp and the read/writeheads are loaded onto the disk surface at a controlled speed. At step703, a predetermined VCM current is used to bias the actuator againstthe ID crash stop and made ready for the SSW process. The samepredetermined VCM current used for biasing the actuator against the IDcrash stop is used throughout the SSW process. At step 704, burstpatterns are written using a predetermined bias voltage applied to themicroactuator. Usually, 100-200 bursts are written per one diskrevolution. For example, if the track has 200 sectors (sectors 0-199), aburst in written in each of sectors 0-199. The burst write timing andthe VCM current are changed for each servo track. The result of step 704appears similar to the results of the conventional initial track settingtechnique, which is shown in FIG. 4.

After the burst pattern has been written, the head is moved toward theinnermost portion of the disk at step 705 by changing the bias voltageapplied to the microactuator, and burst b0, i.e., the burst patternlocated closest to the ID of the disk, is located using the read sensorof the read/write head. At step 706, the read/write head is moved towardouter diameter by changes in the microactuator bias voltage to locatebursts b1 and b2. At step 707, the read sensor portion of the head ispositioned over the center of burst b1 so that the amplitude of burst b0equals the amplitude of burst b2 and the amplitude of burst b1 is amaximum. At step 708, the respective amplitudes of bursts b0, b1 and b2are measured during several disk revolutions and averaged.

At step 709, the overlap is calculated, defined as Overlap=(b0+b2)/b1,in which b0, b1 and b2 are the respective averaged amplitudes of burstsb0, b1 and b2. At step 710, the head is moved toward the OD of the diskby changing the bias voltage applied to the microactuator, measuring andaveraging the amplitudes of each burst b2-b14 and their respectivelyadjacent bursts are measured and averaged. For each measurement in step310, the read sensor portion of the head is positioned over the centerof the burst for which the overlap measurement is being made (i.e., thecenter of each burst b2-b14), so that the amplitudes of the bursts thatare adjacent to the burst being measures are equal and the amplitude ofthe burst being measured is a maximum. The overlap is calculated foreach burst b2-14, similar to calculation for overlap in step 709. Atstep 711, the calculated overlaps are compared to a target overlapvalue, such as 0.9. If, at step 711, the difference between thecalculated overlaps and the target overlap value is within a selectedcriterion, such as 2%, the flow continues to step 712 and the initialtrack-pitch-setting technique is terminated.

If, at step 711, the calculated overlap is not within the selectedcriterion of the target overlap value, then flow continues to step 713where it is determined whether the calculated overlap is greater thanthe target overlap value. If, at step 713, the calculated overlap isgreater than the target overlap value, flow continues to step 714 wherethe predetermined bias voltage increment applied to the microactuator isincreased a predetermined amount. Flow continues to step 716. If, atstep 713, the calculated overlap is less than the target overlap value,flow continues to step 715 the predetermined bias voltage incrementapplied to the microactuator is decreased a predetermined amount. Flowcontinues to step 716 where all previously written bursts are erased.Flow continues to step 703 with the new bias voltage step and theprocess is repeated until the calculated overlap is within the selectedcriterion of the target overlap value.

While technique of setting the initial servo track pitch by initiallypositioning the actuator of the HDD against the ID crash stop, it shouldbe understood that the technique of the present invention can also beused by initially positioning the actuator against the OD crash stop.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced that are within the scope ofthe appended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

1. A method for setting an initial servo track pitch for a servo systemof a hard disk drive having at least one hard disk, an actuator, asecondary actuator, a read/write head corresponding each hard disk andat least one crash stop for the actuator, the method comprising stepsof: (a) positioning the actuator against the crash stop; (b) writing aburst pattern on at least one hard disk while the actuator is positionedagainst the crash stop and the read/write head is in a first position;(c) changing a bias voltage applied to the secondary actuator by apredetermined bias voltage increment for a predetermined number of timesto change the position of the read/write head a corresponding number oftimes and writing a burst pattern on at least one hard disk at eachrespective changed position of the read/write head; (d) determining anamount of overlap for at least one selected burst pattern having twoburst patterns that are adjacent to the burst pattern; (e) terminatingthe method for setting the initial servo track pitch when the amount ofoverlap determined for each selected burst pattern is within a selectedcriterion of a predetermined target overlap value; (f) increasing thepredetermined bias voltage increment when the amount of overlap for eachselected burst pattern is greater than the predetermined target overlapvalue, and decreasing the predetermined bias voltage increment when theamount of overlap for each selected burst pattern is less than thepredetermined target overlap value; and (g) repeating steps (a) through(e) with the new predetermined bias voltage increment.
 2. The methodaccording to claim 1, wherein the step of determining the amount ofoverlap for each selected burst pattern includes steps of measuring anamplitude of a plurality of selected burst patterns, each selected burstpattern having two adjacent burst patterns; measuring an amplitude ofeach burst pattern that is adjacent to each selected burst pattern; anddetermining the overlap for each selected burst pattern as a sum of theamplitudes of the burst patterns that are adjacent to the selected burstpattern divided by the amplitude of the selected burst pattern.
 3. Themethod according to claim 2, wherein the step of measuring the amplitudeof the plurality of selected burst patterns and the step of measuringthe amplitude of each burst pattern are each performed a predeterminednumber of times, the method further comprising steps of: averaging themeasured amplitudes of each respective selected burst pattern; andaveraging the measured amplitudes of each burst pattern that is adjacentto each respective burst pattern, and wherein the step of determiningthe overlap for each respective selected burst pattern is based on theaveraged measured amplitudes of each respective burst pattern.
 4. Themethod according to claim 1, wherein the predetermined number of timesthe position of the read/write head is changed is sixteen.
 5. The methodaccording to claim 4, wherein the overlap is determined for fourteenburst patterns.
 6. The method according to claim 1, wherein the crashstop is an inside diameter crash stop.
 7. The method according to claim1, wherein the crash stop is an outside diameter crash stop.
 8. Themethod according to claim 1, wherein the secondary actuator is amicroactuator.
 9. The method according to claim 1, wherein the secondaryactuator is a milliactuator.